Photo-redox reactions of indole and ferric iron in water

Abstract

Iron-organic interactions are involved in a variety of environmental phenomena, including photo-redox reactions, iron cycling and bioavailability, as well as contaminant fate. In this study we examined UV-induced redox reactions of iron and indole in water. The presence of one indole in the irradiated system resulted in the presence of eight reduced ferric ions, not counting direct photolysis of Fe3+ complexes with OH-, which gives Fe2+ and hydroxyl radical (HO) as products. The main mechanisms that contribute to indole-related Fe3+ reduction i.e. Fe2+ accumulation are: (i) HO scavenging, which prevents oxidation of Fe2+ by HO; (ii) oxidation of indole and its derivatives by excited ferric iron; (iii) reduction of ferric iron by excited indole (not present under UV-A). Hydrated electrons released by UV-B-excited indole play only a minor role in the reduction of iron. Indole-derived radicals emerged as byproducts of indole/iron photo-chemistry. 1H NMR and low-T EPR spectroscopy showed that indole forms a weak low-symmetry complex with Fe3+. The strongest interactions between iron and π-cloud in the indole ring are at positions 2, 3, and 7. The formation of complex promotes electron transfer from excited indole to Fe3+. Our findings are important for understanding the catalysis of photo-reduction of iron by heterocyclic aromatic pollutants, and for the development of protocols for indole processing in wastewaters.